Bluegills, and most of the other species of freshwater "Sunfishes" will eat just about anything they can get in their mouths, and even some things they can't.
I've caught them on big bass flies & lures, usually foul hooked, as they swipe at such a fly or lure.
There are 2 basic types of foam poppers, one is a hard foam, the other a soft foam. There are places that sell both types of finished flies & the bodies to make your own.
I'm guessing from your question, that you have not as yet started tying your own flies? If not & desire to do it, there are some very easy patterns that are great for Bluegills & materials can be obtained in craft shops at very small cost. You'll need some basic tools, a vise (or something similar to hold the hook), a thread bobbin, scissors or other cutting tool, tying thread & hooks. The other materials will depend on the fly patterns.
There are several posts about getting started with tying, so a search should provide some additional information.
However, if you wish to buy flies there are also inexpensive flies that can be purchased.
I tie most of my own flies, but do purchase some occasionally. Check out Millers Fly Shop (Miller's Fly Shop Home Page
) Their flies are decent quality & inexpensive. They also ship at a very fair cost of $2.50 flat rate no matter how many flies you purchase.
There are other such fly suppliers & I'm sure others will chime in with suggestions.
Hope this helps.